Medial temporal lobe. Alzheimer's starts in the hippocampus and the medial temporal lobe. In end-stage alzheimer’s there is widespread atrophy. Early onset alzheimer’s (<65 years old) has a different presentation. There is usually some mild hippocampal atrophy, but the most striking finding is parietal atrophy, with atrophy of the posterior cingulum and the precuneus. The hippocampus can be normal. Hope this helps.